Conventional switching-mode voltage regulators may use analog circuitry. They generally may include a closed loop as shown in FIG. 1 comprising a switching stage controlled by a driver, a low-pass filter (typically a L-C filter) producing a regulated output voltage, a pulse width modulated (PWM) controller and a feedback error amplifier.
The switching stage is usually realized with transistors or SCRs that are switched between a fully conducting state and a non-conducting state, such that the load of the regulator may be powered for a certain Ton time interval and is disconnected from the power supply line for a Toff time interval. The Ton and Toff intervals are typically determined by a PWM control signal, the duty cycle of which is varied as a function of the load for compensating eventual variations and regulating the output voltage.
FIG. 2 illustrates one way to close the voltage loop in a regulator of the pulse skipping comparator type. The output voltage V0 supplied to the load is compared with a reference voltage Vref, and the PWM control signal is either applied or skipped to regulate the output voltage at a desired value Vref. Analog closed loop regulators may be straightforward to realize, but system stability may be obtained using relatively large capacitance values (often not easily integrated on silicon) or using rather complex filtering architectures (for example, switched capacitors filters, sigma-delta converters and digital filters).
An alternative design approach to the analog closed-loop control of a voltage regulator illustrated in FIG. 1 may comprise using a comparator, and in using the comparator output in a LOGIC CONTROL BLOCK for implementing a digital control method based on the so-called “pulse skipping”, or “burst mode” or “constant Ton” technique.
The “pulse skipping” technique comprises regulating the output voltage by supplying the load for a PWM cycle with the maximum allowed duty cycle and successively leaving it unchanged for one or more consecutive PWM pulses as far as the output voltage drops to or below a certain reference voltage Vref. The “pulse skipping” technique also comprises again supplying the load for another PWM pulse at the maximum allowed duty cycle and so on. The fact that the duty cycle maintains the maximum theoretical value may ensure that the regulator produces the desired output voltage regulation with the lowest supply voltage.
A characteristic of the “pulse skipping” technique comprises transient responses being generally faster than in analog closed-loop regulators. This may be because the duty cycle is always fixed at the maximum theoretical value. In addition, the system may be intrinsically stable; there may be need for discrete silicon resistors and capacitors for loop compensation. On a different account, the output voltage ripple is larger than in analog closed-loop regulators for the same reason: the load being powered in a PWM cycle with the largest possible duty cycle. It may be difficult to control a voltage regulator with a pulse skipping technique such as to obtain a fast transient response and a small output voltage ripple.